Ventilation systems comprising more than one ventilation units are well known in the art. However when several ventilation units are operating in relation to the same enclosure the risk of ventilating too much or too little is increased in that controlling the overall effect of all the ventilation units simultaneously becomes very difficult.
From the International patent application WO 95/14890A1 it is therefore known to synchronise the operation of a pair of ventilation units so that a unit interchangeably blows air into a room at one side of a house while the other unit suck air out of the room at the opposite side of the house and visa versa to achieve air exchange equilibrium.
However, if a window in the room is opened, if one of the units is more efficient than the other or if one of the units are more wind loaded than the other, equilibrium is very difficult to establish and the overall efficiency of the ventilation system is reduced.
From the German patent application DE 198 55 056 A1 it is known to control the operation of ventilation units placed in separate rooms by means of signals from a room mounted air quality sensor unit and a portable control unit. However, again the overall efficiency of the ventilation system is reduced if one of the units is more efficient than the other or if one of the units are more wind loaded than the other units and it is difficult to control the overall ventilation if e.g. doors between the rooms are opened so that all the units in practise ventilates the same enclosure.
An object of the invention is therefore to provide for advantageous technique for controlling the operation of a ventilation system comprising two or more ventilation units capable of enabling an air flow to or from the same enclosure.